Because they charge you interest from the day you make a purchase. If you pay off your statemented balance IN FULL they waive the interest - they charge it then remove it. If you pay off you statemented balance in full LESS £1.45 you haven't meant the T&C you agreed to, ergo they don't waive the interested they've told you they were going to charge.

Likewise under normal circumstances TCF isn't remotely relevant. You agree that they can alter the interest rates and credit limit whenever they choose. However the OP is now unable to pay taxes using an AMEX card and is unable to meet his obligation under the contract through no fault of his own. So he has some redress!